What is the coronary sinus?

• A. An opening that returns blood to the right atrium.
• B. A major artery supplying the heart muscle.
• C. A nerve bundle coordinating rhythm.
• D. A chamber of the heart.

Answer: (A)

The coronary sinus is the heart’s main venous drainage channel. It collects deoxygenated blood from the myocardium via the cardiac veins (great, middle, and small) and drains it into the right atrium. It sits in the posterior atrioventricular groove and opens into the right atrium, near where the inferior vena cava and right atrial appendage meet. This makes it a venous opening that returns blood to the right atrium, not an artery, nerve bundle, or chamber.